SPFX - sample paging in sharepoint rest calls using $skiptoken

CountryProjectsDataService.ts

import { SPHttpClient, SPHttpClientResponse } from '@microsoft/sp-http';  
import { ICountryProjectsDataService } from "../ICountryProjectsDataService";
import { ICountryProjects, IUser } from '../../../models';



export class CountryProjectsDataService implements ICountryProjectsDataService {
    
    //paging sharepoint rest api search results
    
    public GetAllProjectsByCountryCode(spHttpClient:SPHttpClient, siteUrl:string, ctryCode:string, pageSize:number = 10, pageNumber:number = 1): Promise<ICountryPIViewModel[]>
    {
        let baseList = "CountryProjects"; 
        
        //start of the current page
        let startItemId:number = pageNumber > 1 ? (((pageNumber-1)*pageSize)) : 0;

        let pagingQueryString = `$skiptoken=Paged%3DTRUE%26p_ID%3D${startItemId}&$top=${pageSize}`;
        let filterQueryString = `$filter=Country/Code eq '${ctryCode}'`;
        let selectQueryString = `$select=*,Country/Title`;
        let expandQueryString = `$expand=Country`;       

        let refURL=  `${siteUrl}/_api/web/lists/getbytitle('${baseList}')/items?${selectQueryString}&${filterQueryString}&${expandQueryString}&${pagingQueryString}`;
        let data = spHttpClient.get(refURL,  
            SPHttpClient.configurations.v1,  
            {  
            headers: {  
                'Accept': 'application/json;odata=nometadata',  
                'odata-version': ''  
            }  
            })
            .then((response: SPHttpClientResponse): Promise<{ value: ICountryProjects[] }> => {
                return response.json();
            })
            .then((response: { value: any[] }): ICountryProjects[]=>{

                let countryProjItems: ICountryProjects[] = [];
                if (response) 
                {
                   
                    for (let i = 0; i < response.value.length; i++) 
                    {
                            //do some operations here
                            
                            //push item to collection
                            countryProjItems.push(listItem);                        
                    }
                    
                }
                return countryProjItems;
                
            });         

        return data;

    }     

}